1. Field of the Invention
This invention relates to a recording apparatus for a video signal having a DTF (Dynamic Track Following) function to match a step difference between heads in the recording mode. A pilot signal recorded by one head is reproduced by another head which records the signal in the following track.
2. Description of the Prior Art
As a recording apparatus for a video signal, a recording apparatus for a video signal having a DTF function is already disclosed in published Japanese Patent Application No. 54-115113, in which a pair of heads, mounted on a rotary head assembly with an angular spacing of about 180.degree. therebetween, are attached to electrostrictive elements, such as a bimorph leaf plate and so on. Thus, when recording a video signal, a step difference between the heads in the recording mode is matched while a pilot signal, recorded by one head, is being reproduced by the other head which records the signal in the next track.
FIG. 9 illustrates an example of a rotary head assembly of the prior art used in such a recording apparatus. This is a format used in unitary type video, such as a so-called 8 mm video tape recorder, and so on.
In drawing FIG. 9, reference numeral 1 designates a rotary drum on which a pair of recording/reproducing heads H.sub.1 and H.sub.2 which are different in azimuth angle, are mounted with an angular spacing of substantially 180.degree. therebetween. These heads H.sub.1 and H.sub.2 are attached to electrostrictive elements 3 and 4, such as bimorph leaf plates. A magnetic tape 2 is wound around the rotary drum 1 over a predetermined angular range larger than 180.degree.. A track corresponding to a tape wrapping angle exceeding 180.degree. is used as an optional track (used for recording a PCM audio signal and the like).
FIG. 10 shows a prior art tape format thereof, in which T.sub.0 represents an optional track and T.sub.V represents a video track. In order to match the step difference between the heads upon recording, prior to the recording of a video signal, a pilot signal S.sub.P used to match the step difference between the heads is recorded on every track over a predetermined interval W.sub.R (within 3 H) by using the heads H.sub.1 and H.sub.2. A part of an optional track is assigned to the predetermined interval W.sub.R.
The pilot signal S.sub.P recorded by the head H.sub.1, is reproduced by the head H.sub.2 used to form the next track T.sub.V, while the pilot signal S.sub.P, recorded by the head H.sub.2, is reproduced by the head H.sub.1 used to form the next video track. Accordingly, as to each of the heads H.sub.1 and H.sub.2, the interval W.sub.R is the recording interval of the pilot signal S.sub.P, while the succeeding interval W.sub.P is the reproducing interval of the pilot signal S.sub.P.
Accordingly, if a drive voltage is applied to the electrostrictive elements 3 and 4 with the head H.sub.1 or H.sub.2 attached in such manner that the crosstalk component of the pilot signal S.sub.P reproduced by the head H.sub.2, and the crosstalk component of the pilot signal S.sub.P reproduced by the head H.sub.1 become equal, the step difference of the head H.sub.2 relative to the head H.sub.1 always becomes constant. Thus, a constant step difference is maintained between the heads during the recording mode at all times.
FIG. 11 illustrates a relatioship among reproduced outputs (RF outputs) for first and second channels CH-1 and CH-2 derived from the heads H.sub.1 and H.sub.2, and a head switching pulse P.sub.SW when such a DFT control is carried out.
For the tracking in the playback mode, there is used a tracking servo which employs a pilot signal having 4 frequencies. In other words, when a video signal is recorded on the first video track T.sub.V, a pilot signal of frequency f.sub.1 is recorded thereon in a superimposed state, and in like manner, respective sine wave signals of frequencies f.sub.2, f.sub.3, and f.sub.4 are recorded on the second to fourth video tracks T.sub.2 to T.sub.4 as the pilot signals inherent in the tracks. With these relationships, pilot signals corresponding to succeeding video tracks are recorded sequentially.
By way of example, frequency relations of a plurality of pilot signals are selected as follows (see FIG. 12):
f.sub.1 =102 kHz PA1 f.sub.2 =118 kHz PA1 f.sub.3 =165 kHz PA1 f.sub.4 =149 kHz
When the pilot signal S.sub.p is recorded and reproduced by the recording apparatus having the DTF function, the playback interval W.sub.P in which the pilot signal S.sub.P can be reproduced does not include the entire recording interval W.sub.R of the pilot signal S.sub.P, but it is determined by an interval T.sub.X in which the beginning of the track is displaced between the adjacent tracks regardless of the length of the recording interval W.sub.R.
That is, as shown in FIG. 13, if a start point at which the pilot signal S.sub.P (hereinafter referred to as S.sub.P1) is recorded by the first head H.sub.1 which traces the track T.sub.1, is taken as a, and an end point thereof is taken as d, the interval from a to d becomes the recording interval W.sub.R. The length of this interval is determined as 3 H as a maximum. If the displacement between the beginnings of the tracks T.sub.1 and T.sub.2 is taken as T.sub.X, the pilot signal S.sub.P (hereinafter referred to as S.sub.P2) that is recorded by the second head H.sub.2 which traces the track T.sub.2 becomes the same as the pilot signal S.sub.P1 in the recording start timing. Thus, the recording start point of the pilot signal S.sub.P2 becomes a time point b which precedes the time point a by T.sub.X, and a time point c which is distant therefrom by W.sub.R becomes the recording end point.
On the other hand, the second head H.sub.2 which traces the second track T.sub.2 is set in the playback mode from the recording end point c of the pilot signal S.sub.P2. Thus, the crosstalk component of the pilot signal S.sub.P1 recorded on the first track T.sub.1 is reproduced thereby. Consequently, the playback start point of the pilot signal S.sub.P1 becomes the point c, and the end point thereof becomes the point d, so that the interval from c to d becomes the playback interval W.sub.P of the pilot signal S.sub.P1.
Since the interval bd in the first track T.sub.1 becomes equal to (W.sub.R +T.sub.X), and the interval bd in the second track T.sub.2 is equal to (W.sub.R +W.sub.P), the playback interval W.sub.P of the pilot signal S.sub.p1 is equal to the track displacement T.sub.X between the adjacent tracks. Accordingly, when W.sub.R =3 H and T.sub.X =1.5 H, even if the recorded interval W.sub.R is 3 H, the interval W.sub.P in which the pilot signal S.sub.p1 can be reproduced practically becomes 1.5 H. In such a case, even when the recording interval W.sub.R is selected to be 1.5 H, no troubles occur so that as shown in FIG. 14, W.sub.R -1.5 H (.thrfore.W.sub.P =1.5 H) is established.
Such a relationship is also established for a case in which the pilot signal S.sub.p2 recorded on the second track T.sub.2 is reproduced by the first head H.sub.1 which traces the first track T.sub.1 which is adjacent.
If the playback interval W.sub.P of the pilot signals S.sub.p1 and S.sub.p2 is presented as about 1.5 H, the crosstalk components thereof can be detected easily so that no troubles will occur in producing a voltage which drives the electrostrictive elements 3 and 4.
However, in the case of a recording apparatus such as an 8 mm video tape recorder or the like in which the track displacement T.sub.X is selected to be less than 1.5 H, such as 1.0 H or 0.5 H, in consideration of the head H alignment, there is the defect that as the displacement T.sub.X becomes small, the playback interval W.sub.P also becomes shorter. In this case, T.sub.X -0.5 H is for an LP (Long Play) mode, while T.sub.X -1.0 H is for an SP (Short Play) mode, respectively.
FIG. 15 shows an example of the recording and playback intervals W.sub.R and W.sub.P when T.sub.X -0.5 H is established. The playback interval W.sub.P of the pilot signals S.sub.P1 and S.sub.P2 becomes 0.5 H even when the recording interval W.sub.R is 3.0 H.
If the playback interval W.sub.P becomes short as described above, the playback information amount is extremely reduced so that the drive voltage for the electrostrictive elements 3 and 4 cannot be generated easily. When generating the voltage, a band pass amplifier having a good S/N ratio must be used, which causes the manufacturing cost to be increased.